1. Field of the Invention
The present invention relates to a projection cathode ray tube, and more particularly, to an improved mounting for the target member of the tube.
2. Description of the Prior Art
Projection cathode ray tubes that project electronically generated images upon a viewing screen external to the tube are well known in the art. Traditional components of such a tube include an electron gun for providing an electron beam, a target coated with a material responsive to the electron beam to produce an image, concave mirror for reflecting and amplifying the image, a target support assembly holding the target fixed with respect to the mirror, and a transparent face plate through which the amplified image is projected from the mirror.
In all uses of a projection cathode ray tube, a continuously focused and sharp image is desired. For example, in entertainment uses a sharp image assures viewing pleasure. In industrial information transfer applications, a sharp image is critically important to creating a life-like picture. One industrial application in which such a sharp and life-like image is required is in the training environment of flight simulators. The image presented to a trainee should desirably be so sharp and life-like that the trainee experiences the flight simulator as an actual flight in an aircraft.
For these reasons, significant attempts have been made to improve image quality of projection cathode ray tubes. However, conventional tubes are subject to a condition known as "hot focus drift", produced in the tube as follows. The tube heats during use. The target is heated most of all because of the electron beam hitting the target. This condition causes the material of the target to expand, and leads to undesirable changes in target distance with respect to the mirror. As a consequence of this electron beam heating of the target, and resultant linear displacement of the target face, the image becomes defocused. Angular displacement of the target, called "target tilt," is another undesirable condition causing misalignment of the image with respect to the tube's face plate. Target tilt may be caused by an accidental jolt during handling in transportation, use or service, or from deformation of the target because of differential thermal expansion between the target and its support structure.
A target assembly developed to dissipate heat and for angular stability against target tilt is shown in U.S. Pat. No. 4,177,400 to Hergenrother et al. A target support shaft extends through the face plate of the projection tube and is held in place by the combination of a compression spring extending between the target and the face plate interior on one side of the face plate and a slip-joint mounting pad on the outside of the face plate. The target has a threaded mounting bore receiving a threaded target stud portion of a connecting spindle. The spindle has a threaded shaft stud opposite the target stud, and the target is thus connected to the support shaft when the shaft stud is threaded into the support shaft. Angular adjustment of the target is achieved by the rotating of mounting pad set screws bearing against the face plate, thus changing the angle of the support shaft as it is held in tension by the compression spring. While this arrangement worked well enough to become the standard in the art, its slip-joint mounting pad structure proves inadequate to protect against linear or angular displacement of the target caused by even slight external knocks or impacts.
Further, heat-induced deformation of the target material is a continuing problem. Specifically, the compression spring bears on the target directly, and the target stud threaded into the target mounting bore has a spindle shoulder flange abutting the bore. The portion of the target that is located between this flange and the threads on the target stud expands and will deform when the fastener load exceeds the yield strength of the target material. When the target cools and contracts, the compression spring can displace and tilt the target by an amount as large as the clearance that remains after cooling between the deformed target portion and the spindle shoulder flange.
The design shown in U.S. Pat. No. 5,204,751 to Salyer et al. improved support shaft rigidity by employing external threads on the shaft to fasten the shaft to the internally threaded mounting pad, replacing the former slip-joint connection. In addition, a locking nut on the shaft loads the cooperating shaft-pad threads of the shaft and mounting pad to further inhibit movement at this connection. For additional rigidity, a set screw is threaded into the hollow shaft and bears against the shaft stud, loading the cooperating threads of the shaft and the shaft stud. Although the Salyer et al. patent apparatus provides increased shaft rigidity, the arrangement of a hollow shaft with a set screw is somewhat complex and not optimal for heat dissipation. Also, deformations caused by heating of the target material coupled with direct pressure from the compression spring remain as undesirable effects of normal operation of the tube.
Accordingly, it is an object of the present invention to provide a projection cathode ray tube having a target support assembly that avoids or minimizes the above mentioned problems.
It is a specific object of the present invention to provide a target assembly that controls target tilt by a tapered interface design, allowing the target to expand and contract freely with minimal deformation under electron beam heating.
It is another object of this invention to minimize target tilt by relocating spring-loading of the target assembly from the target to the support shaft.
It is yet another object of this invention to minimize hot focus drift through improved design of the target and target support assembly configuration and employment of low thermal expansion materials for the support shaft.